Increasing eutrophication driven by the increase of phosphate discharge in a subtropical bay in the past 30 years

Abstract

Although great efforts have been made to decrease nutrient (notably nitrogen and phosphorus) loads and lighten related environmental damages, coastal eutrophication remains a persistent environmental crisis. To estimate whether the rapid development of the local economy has accelerated the eutrophication in Zhanjiang Bay, a newly developing industry in South China, the combination of the seasonal nutrients and other physicochemical parameters from twenty cruises during 2017-2021 and historical data (past 30 years) was analyzed in this study. The results showed that the eutrophication in the upper bay is significantly higher than that in the lower bay (more than 8 times), which is mainly related to the terrestrial input and weak hydrological conditions in the upper bay. Also, eutrophication is more severe in the rainy seasons than that in the dry seasons (nearly 2 times) because abundant nitrogen and phosphorus were brought into the bay by terrestrial discharge and river water. From a long-term perspective, dissolved inorganic nitrogen concentrations have been effectively controlled in Zhanjiang Bay, while phosphate concentration increased sharply in the past 30 years. Correspondingly, the eutrophication in Zhanjiang Bay significantly increased over the past 30 years, and the eutrophication index in the recent 5 years (2017-2021) is nearly 10 times that of the 1990s, suggesting that the increase of phosphate discharge from the increasing industrial factories around Zhanjiang Bay, rather than nitrogen discharge is the culprit causing the aggravation of eutrophication. Our study is essential to effectively implement a land-ocean integrated nitrogen and phosphorus control strategy to improve water quality and mitigate eutrophication in the bay.

Changes in hydrodynamics and nutrient load of the coastal bay induced by Typhoon Talim (2023)

“…Compared to the Typhoon Barijat period (an average of 45.2 mmol L -1 for DIN) (Lao et al, 2023c), the stronger ocean front induced by the Typhoon Talim resulted in a significant increase in DIN concentration in the upper bay (increased by nearly 50%). This is mainly because runoff is the main DIN source in the bay, and the DIN input exhibited an increasing trend in the recent five years (2017-2021) (He et al, 2023). On the contrary, the input of PO 4 3decreased during this period (He et al, 2023), and thus less change of PO 4 3contribution was observed after the Typhoon Talim compared to the Typhoon Barijat (an average of 3.5 mmol L -1 in the upper bay).…”

“…This is mainly because runoff is the main DIN source in the bay, and the DIN input exhibited an increasing trend in the recent five years (2017-2021) (He et al, 2023). On the contrary, the input of PO 4 3decreased during this period (He et al, 2023), and thus less change of PO 4 3contribution was observed after the Typhoon Talim compared to the Typhoon Barijat (an average of 3.5 mmol L -1 in the upper bay). However, in the lower bay, the DIN concentration during the posttyphoon Talim decreased compared to the Typhon Barijat (an average of 11.8 mmol L -1 ) (Lao et al, 2023c), mainly influenced by more seawater intrusion after stronger typhoon Talim.…”

“…This has caused devastating damage to local aquaculture activities (Zhou et al, 2021). The rainy season is the most frequent period of typhoon activity in Zhanjiang Bay, which is likely an important factor in aggravating eutrophication ( Zhang et al, 2021;Zhang et al, 2022;He et al, 2023). According to historical data, eutrophication in Zhanjiang Bay has shown a significantly increasing trend over three decades, especially during the rainy seasons (He et al, 2023).…”

“…The rainy season is the most frequent period of typhoon activity in Zhanjiang Bay, which is likely an important factor in aggravating eutrophication ( Zhang et al, 2021;Zhang et al, 2022;He et al, 2023). According to historical data, eutrophication in Zhanjiang Bay has shown a significantly increasing trend over three decades, especially during the rainy seasons (He et al, 2023). This has led to an increase in the frequency of harmful algal blooms in Zhanjiang Bay (Zhang et al, 2022), posing a huge threat to the local eco-environment and aquaculture activities.…”

Unveiling the eutrophication crisis: 20 years of nutrient development in Zhanjiang Bay, China

“…Moreover, excessive nutrient levels may cause significant phytoplankton blooms and high primary productivity, resulting in eutrophication, red tides, algal blooms, water hypoxia, and acidification (Wang et al, 2021a;Zhang et al, 2023aZhang et al, , 2023b. These factors can degrade the affected environments, directly threaten biodiversity and habitats, and cause massive mortality of marine organisms, ultimately harming marine ecosystems and human society (Deegan et al, 2012;Mauŕe et al, 2021;He et al, 2023).…”